Phyto-purification or lagooning: functioning pros cons costs

The phytoépuration is the generic name for all the ecosystems capable of ridding a site soiled by organic, mineral or, to some extent, chemical pollution. More commonly intended for industrial use or for equipping small and medium-sized communities, phytopurifiers can constitute ecological solutions accessible to autonomous private sanitation.

In this dossier, we will deal more specifically with the treatment of domestic wastewater by lagooningpractical application on the model of natural wetlands.

Legality of non-collective sanitation by lagooning

This equipment does not benefit fromno approval health authorities. Before even giving the first pickaxe, or filing a building permit, the user must imperatively make sure to fulfill two conditions:

  • Check that no network sewage does not pass by, because the law makes obligation owners to connect their buildings to these collectors within a maximum of 2 ans after their commissioning.
  • Obtain derogatory authorization prior to installation with the Departmental Service of the Water Police (SDPE). A serious study and the use of the Non-collective Public Sanitation Service (SPANC) of the municipality, can provide decisive assistance in supporting this type of case.

Principle of sanitation by lagooning

The combined action of microorganisms, suspended plants and aquatic plants, transforms the pollution transported by wastewater into a complete food chain, extending from bacteria to fish. The lagooning site is a set of artificial basins (fell apart) intended to improve the concentration and fixation of these nutrients in order to create a confined environment conducive to the growth of microscopic flora and fauna. The circuit and the flow time are specifically studied to optimize each stage of the treatment. At the end of the loop, the water must be sufficiently purified to be infiltrated if the permeability of the ground is sufficient. Evacuation to a pond, a watercourse or a ditch may constitute a last resort, subject to study and authorisation. The reuse for domestic or industrial use, follows the same procedure.

Physiognomy of a lagoon sector

After pre-treatment, the flow of water to be treated crosses at slow speed (between 1 week and 1 month), a network of at least 3 lagoons hydraulically connected by pipes or by a spilling edge. The basins have different characteristics depending on their level, the type of waste and the flow of the tributaries. They can be built of different materials, simply dug in clay soil, coated with a layer ofcompacted clay or a geomembrane, but they are still waterproof. The basins are organized in a hierarchical sector according to their function. Rectangular lagoons offer better performance.


First wastewater treatment, the screen mechanically retains heavy, bulky or difficult to biodegrade elements (sand, pebbles, bottles, metal objects, wipes, etc.). It is essential to prevent the obstruction of pipes or the clogging of settling tanks. It is also life insurance for any pumps located downstream.

Maintenance: vclean the receptacle 3 to 4 times a year.


The coarse oil removal of the effluents is carried out by means of a grease trap of suitable dimensions. Rarely implemented in collective facilities, it is becoming essential for private facilities in order to reduce the land area of ​​the sector. It is compulsory for installations equipped with all-water pits located more than 10 meters from buildings or dedicated to the catering professions.

Maintenance : skim the surface layer 2-3 times/year.

First lagoon: settling

The lagoon of settling occupies more than half of the lagoon space. It is a deep buffer basin, in which heavy solids are transformed:

  • In gaz (Carbon dioxide, Methane, Hydrogen sulfide, etc.), before dispersing in the ambient air and/or diluting in water.
  • In mineralsprocessed over the course of the following basins.

These elements are the residues of deep sludge digestion by microorganisms anaerobes, naturally present in the environment. The performance of installations can be improved by increasing the density of useful species to the detriment of those whose abundance creates malfunctions or generates unpleasant odors. These crops are highly dependent on local climatic conditions and the composition of the waste to be treated. On leaving the first basin, the water has lost up to 75% of its impurities.

Maintenance : pump the sludge every 7 to 10 years.

Alternatives to settling ponds

Some lagoon systems do away with the settling lagoon, entrusting all the degradation processes to microorganisms in the same basin, aerobic and some at the surface, and anaerobic at depth. These devices require very grandes surfaces land, times of long treatment and are energy consumers for oxygenation by stirring.

In individual installations, space savings often encourage the replacement of the settling tank by a septic tank, providing the same service. The lagoon system is then reserved solely for final biological and mineral purification.

Second lagoon: biological and mineral treatments

The second lagoon is shallower and of smaller surface. Its role is to complete organic cleaning and to rid the water of most of its pathological microbial (coliforms, faecal streptococci, etc.) and mineral (nitrates, phosphorus, etc.) loads. Part of the oxygenation is ensured by a flora of microscopic algae (phytoplankton) feeding on minerals and decomposition gases, hence the predominantly green color of the basin. In addition, sunshine and wind promote oxygenation by photosynthesis et brewing. These algae and bacteria in turn serve as a meal for the upper echelon, the zooplankton. Deep mud serves as a fixative for plants and mineralizes slowly.

Maintenance : limited to maintaining the banks and controlling bacterial loads.

Third lagoon: The finishes

At this stage, the water is clear. Zooplankton and plants take the last samples of pollutants, maintaining the life of a host of living filters (amphibians, crustaceans, insects, etc.). The edges of this landscaped lagoon are planted plants macrophytes (rushes, bulrushes, papyrus, etc.) whose roots in turn participate in the oxygenation of the water.

Residual water can be infiltrated, channeled to new inhabited ornamental ponds (amenity fish) or recovered (watering, breeding, fish farming, etc.).

Maintenance : Weeding and water quality monitoring. Ducks and other birds are responsible for thinning the plantations and regulating the aquatic fauna.

Advantages of lagoon systems

  • Eco-friendly, eco-friendly ecosystem
  • Landscaped integration into the site
  • Higher decontamination efficiency than most conventional systems
  • Moderate investment
  • Low operating costs, low maintenance
  • Great durability

Disadvantages of lagoon systems

  • Requires large areas of land.
  • Administrative derogation.
  • Operation depends on the seriousness of the preliminary study.
  • Yield varies with the seasons.
  • Quick but regular maintenance.
  • May attract rodents and mosquitoes.

Construction costs

The investment cost depends on its treatment capacity, the conformation of the land and the landscape finish requirement.

Count, on average:

  • in between 3 500 et 5 000 € TTC for the sanitation of a family house with 5 rooms.
  • In between 500 et 1 000 € TTC for the preliminary study.

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